Method of, and apparatus for, controlling the crystalline structure of alloys, and alloys so produced

ABSTRACT

The crystalline structure of an alloy having a structure which is at least partially eutectic is controllable by heat-treatment, by forming in a body of the alloy a zone of high temperature gradient between a melting region and an adjacent solidification region. The rate of progress of said high temperature gradient over the body may be controlled whereby to influence a predetermined crystallization in the matrix of the eutectic.

United States Patent Sahm et al. 1 June 24, 1975 [54] METHOD OF, AND APPARATUS FOR, 2,739,088 3/1956 Pfann 75/65 ZM 2,855,335 10/1958 Seiler el al... 75/65 2M 3,030,194 4/1962 Emeis 75/65 ZM STRUCTURE OF ALLOYS AND ALLOYS S 3,060,123 /1962 Theurer 75 ZM PRODUCED 3,124,452 3/1964 Kraft /65 [75] Inventors: Sahm, Nussbaumen; Rema 3,496,033 2 1970 G1lbreath,.lr. et 148/13 R Seba|j N0vosel Neuenhof both of 3,498,851 3/1970 Shuin et al 148/103 S 1 d 3,536,540 10/1970 Rauch 148/13 R 3,552,953 1/1971 Lemkey et a1. 75/135 73 Assignee; Akti ll h fl Brown, B -i & 3,564,940 2/1971 Thompson et al 75/171 Cie., Baden, Switzerland Primary Examiner-Winston A. Douglas [22] Filed 1972 Assistant Examiner-Mark Bell [21] Appl. No.: 244,652 Attorney, Agent, or FirmOblon, Fisher, Spivak,

McClelland & Maier [30] Foreign Application Priority Data May 10, 1971 Switzerland 6882/71 [57] ABSTRACT I The crystalline structure of an alloy having a structure [52] 7 823754 23 which is at least partially eutectic is controllable by j heat-treatment, by forming in a body of the alloy a [51] 'f gg g f gg i 14 zone of high temperature gradient between a melting 1 Fla d 0 earc 75 is a 5 region and an adjacent solidification region. The rate I of progress of said high temperature gradient over the body may be controlled whereby to influence a prede- [561 uNlTE g gg xg gs gi TENTs termined crystallization in the matrix of the eutectic. 2,719,799 10/1955 Christian 75/65 ZM 3 Claims, 5 Drawing Figures za s Cr 3 C 5 3 2 Fr 10 L Cr C537 COX ,Cl'23 CO C 60 C1 7, CO C3 b) 40 1 METHOD OF, AND APPARATUS FOR, CONTROLLING THE CRYSTALLINE STRUCTURE OF ALLOYS, AND ALLOYS SO PRODUCED The invention relates to a method of controlling, by heat treatment, the crystalline structure of alloys which have a structure which is at least partially eutectic. The invention also includes an apparatus for carrying out the method, and also a determined application of the method in the cases of certain alloy compositions.

Because of their widespread availability and their properties, alloys which have a structure which is at least partially eutectic are of great technological interest. The same therefore also applies to the heat treatment of such alloys with the object of directing their structure towards determined improvements of properties. The aim ofsuch improvements is in many cases the attainment of a uniform and/or symmetrical crystalline structure of the eutectic. particularly in respect of the shape and arrangement of the crystals embedded in the matrix of the eutectic. The problem underlying the invention therefore consists in providing a method of heat treatment and an appertaining apparatus or a corresponding application of the method whereby structure control of the kind mentioned can be achieved.

The method according to the invention for the solution of the problem is characterized in that a zone with a high temperature gradient between a melting region and a solification region is produced in the alloy. Taking as starting point an apparatus for the treatment of workpieces of an alloy having a partly, or wholly, eutectic structure, the apparatus according to the invention for the solution of the problem is characterized in that a heating apparatus acting on the workpiece and a cooling apparatus. likewise acting on the workpiece are disposed a short distance apart so as to form a transition region with a high temperature gradient. Furthermore, according to the invention a method of the kind described above can be applied by forming the zone having a high temperature gradient between a melting zone and a solification zone in an alloy the eutectic portion of which has a first. at least substantially isotropic. phase and a second. comparatively highly anisotropic. crystalline phase.

Further features and advantages of the invention will be seen from the following description of the examples of embodim cm. which is given with reference to the appended drawing in which:

FIG. 1 is a diagrammatical longitudinal section of an apparatus for the continuous heat treatment ofa workpiece in wire form;

FIG. 2 is a partial longitudinal section in the region II in FIG. 1, on a larger scale;

FIG. 3 shows a part of a ternary phase diagram for a CoCr-C system;

FIG. 4 shows a microscopic polished section of an alloy from the diagram shown in FIG. 3, produced by the conventional method; and

FIG. 5 shows a corresponding polished section after continuous treatment with a high temperature gradient.

In an apparatus according to FIG. 1 a workpiece l, which is in wire form and is surrounded by a thinwalled. high-temperature resistant tube 2 in the manner illustrated in FIG. 2, is inserted into a holder of the push-rod 3 of a driving device indicated by a double arrow 4. Despite the relative movement, the drive device is sealed by means of a bellows 5 in relation to the gas-carrying system of the apparatus, which system comprises a bell 6, an inlet pipe 7, and an outlet pipe 8. By means of these pipes the workpiece is bathed in an inert gas. for example argon, in the region of the heating and melting zone.

The sheathed workpiece 1 passes in the downward direction through the passage of a heating device 11 and, by way of an intermediate space 16, penetrates into a cooling bath 17. The heating device 1], for example a high frequency induction heater, comprises a winding 12 together with a susceptor 14 which surrounds the workpiece, and which for example is made of tantalum, and an outer heat shield l3, made for example of alumina. The heating device can be raised and lowered by means of an adjusting device, indicated diagrammatically by a double arrow 15, whereby the height of the intermediate space 16 is varied accordingly. For a given temperature difference between the outlet of the heating device ll and the inlet of the cooling bath 17, the temperature gradient acting in the intermediate space 16 can thus be adjusted.

With the aid of suitable measuring and control means for temperature and distance, control is also optionally possible for the purpose of maintaining a predetermined ratio between, on the one hand, the difference in temperature between the heating device and the cooling device, and, on the other hand, the distance between these two devices. In the case of the example, the temperature of the cooling bath, which latter may conveniently consist of a molten bath, for example. of a eutectic alloy having a correspondingly low melting point, is controlled additionally. For this purpose a current of a heat transfer medium, for example water. is provided in a cavity which surrounds the cooling bath l7 and is formed between an outer tube 9 and an inner tube 10 and which is provided with connecting pipes 20 and 21.

In a particularly simple and therefore convenient manner the height of the intermediate space and, consequently, if the temperature difference remains unchanged, the value of the temperature gradient between the melting zone and the solidification zone can be adjusted by raising or lowering the level of the cooling bath or, in conjunction with suitable measuring and control elements, can also be automatically controlled. A suitable inlet and outlet device for the cooling medium is indicated in FIG. 1 by the double arrow 19 at the cooling medium connection 18.

With the aid of the illustrated apparatus it is possible to obtain a zone of high temperature gradient which progresses over the volume of the workpiece, that is to say, in the case of the example in the lengthwise direction of the wire, in accordance with a predetermined rate of speed of solidification. At the same time the temperature gradient can be adjusted or automatically controlled with the aid of the devices described.

The above-described treatment with a high temperature gradient has been found particularly effective for alloys with a eutectic which has a crystalline carbide phase embedded in a metallic matrix. The relative region of a phase diagram of a ternary system of this kind is shown in FIG. 3. This relates to a CoCr-C system with a carbide phase of the variable composition Cr Co,C in accordance with region a, and also with a metallic matrix of the composition Co, ,,-,,Cr ,-C,, in accordance with region 11. A eutectic channel e extends between these regions. Investigations have shown that the symmetrizing effect or the field of application of the method requires the presence of an at least substantially isotropic crystalline phase and a relatively highly anisotropic crystalline phase in a eutectic.

In the special system mentioned above as an example, values of the temperature gradient above about 2500Clcrn have been found particularly favorable. and the effect was also observed up to values of l0.000C/cm. so that no restrictive limitation in the upward direction can be seen. On the other hand. in the case of the example the effect of morphological structure symmetrization was observed at speeds of solidification between and 100 cm/hour. Relatively high values of speed of solidification or speed of passage of a workpiece can therefore be attained, while small diameters or layer thicknesses in the case of workpieces in wire or strip form facilitate the attainment of a high temperature gradient.

A particularly clearly defined symmetrizing effect was obtained with a ternary system according to FIG. 3 within a range embraced by the following corner compositions:

An example of the attainable structure symmetrization can clearly be seen from comparison of the microscopic polished sections shown in FIGS. 4 and 5, in respect of both the shape and the arrangement of the carbide crystals (dark areas) in their statistical distribution inside the matrix (light ground).

Therefore. whereas in the untreated condition the carbide phase solidifying in fiber form has a very irregular formation and distribution. thus impairing the advantageous properties of the alloy. through the treatment with a high temperature gradient and the structure symmetrization achieved there is obtained better high temperature resistance to structure coarsening and consequently improved fatigue strength. Consequently it is now possible to make full use of the inherently good properties of the alloy in respect of high temperature strength and high temperature corrosion resistance.

We claim:

1. In a method of controlling by heat treatment the crystalline structure of an at least partially eutectic alloy having a crystalline (Cr, Me) C;, phase embedded in a Me-chromium-carbon matrix phase, wherein Me is selected from the group consisting of Fe, Co, Ni, Mn and combinations thereof, wherein one of said phases is at least substantially isotropic, and the other phase is characterized by a highly anisotropic crystalline structure. the improvement comprising symmetrizing said structure by passing said body through a melting zone and a solidification zone, wherein the temperature gradient between the melting zone and the solidifcation zone is at least 2.500C/cm, and wherein the speed of said body is advanced through said zones is a rate of from l-lOOcm/hr.

2. The method of claim I, wherein said metal is cobalt and wherein the total composition of the resulting cobalt-chromium-carbon alloy lies within the following corner compositions in percentages by weight:

1) Co=53.l K Cr=44.7% C=2.21 2) Co 50.25% Cr 47.0% C 2.75% 3) Co 67.15% Cr 30.0% C 2.85% 4) Co 62.5 71 Cr 34.0% C 3.5 7:

3. The method of claim 1, wherein the upper limit of the temperature gradient is l0,000C/cm. 

1. IN A METHOD OF CONTROLLING BY HEAT TREATMENT THE CRYSTALLINE STRUCTURE OF AN AT LEAST PARTIALLY EUTECTIC ALLOY HAVING A CRYSTALLINE (CR,ME)-C3 PHASE EMBEDDED IN A ME-CHROMIUMCARBON MATRIX PHASE, WHEREIN ME IS SELECTED FROM THE GROUP CONSISTING OF FE, CO, NI, MN AND COMBINATIONS THEREOF, WHEREIN ONE OF SAID PHASE IS AT LEAST SUBSTANTIALLY ISOTROPIC, AND THE OTHER PHASE IS CHARACTERIZED BY A HIGHLY ANISOTROPIC CRYSTALLINE STRUCTURE, THE IMPROVEMENT COMPRISING SYMMETRIZING SAID STRUCTURE BY PASSING SAID BODY THROUGH A MELTING ZONE AND A SOLIDIFICATION ZONE WHEREIN THE TEMPERATURE GRADIENT BETWEEN THE MELTING ZONE AND THE SOLIDIFICATION ZONE IS AT LEAST 2,500*C/CM, AND WHEREIN THE SPEED OF SAID BODY IS ADVANCED THROUGH SAID ZONES IS A RATE OF FROM 1-100CM/HR.
 2. The method of claim 1, wherein said metal is cobalt and wherein the total composition of the resulting cobalt-chromium-carbon alloy lies within the following corner compositions in percentages by weight:
 3. The method of claim 1, wherein the upper limit of the temperature gradient is 10,000*C/cm. 